The present invention is generally directed to processes for the preparation of encapsulated toners, and more specifically to in situ processes for the preparation of encapsulated toner compositions, wherein the toner particles can be formed in an aqueous medium, recovered from the aqueous medium in the form of a dry powder and then surface treated to impart electrical, release, flow and other desirable toner properties. More specifically, the present invention is directed to the treatment of known encapsulated toner compositions wherein a pressure fixable soft core is enclosed by a pressure rupturable hard shell after one or more reaction steps in an aqueous medium. In one embodiment of the present invention there are provided processes for the drying, that is for example removal of residual water, from encapsulated toners or toner cakes, in economical conventional drying devices, such as a fluidized bed dryer or a vacuum dryer, and the subsequent surface coating thereof in, for example, a high shear blending device with additives which can assist in enhancing the toner imaging performance, and provide conductive encapsulated toners. Moreover, with the process of the present invention in embodiments thereof the use of costly graphites can be avoided, rather known conductive carbon blacks, such as CARBON BLACK BP (black pearls) 2000.TM. available from Cabot Corporation, and the like can be selected. In the known costly spray drying processes for the preparation of encapsulated toners there is usually introduced into a hot air stream, through a nozzle or a disc atomizer, an aqueous suspension of the appropriate solid particles that will provide an encapsulated toner with a surface coating substance, such as certain graphites like AQUADAG E.RTM., and whereby substantial water, for example from about 60 to about 70 percent, is evaporated to enable the encapsulated toner. A number of advantages are associated with the processes of the present invention in embodiments thereof, such as lower thermal energy consumption, for example, there is utilized heated air at lower temperatures, since less water, 10 percent for example, has to be removed during the drying step; and the process is more economical in that, for example, surface additives such as carbon black, or certain powdered forms of graphite can be utilized as the surface additives.
The drying and dry blending processes of the present invention in an embodiment relates to the preparation of encapsulated toners, including in situ toners with surface additives of carbon black and metal salts of fatty acids, such as zinc stearate. A number of processes for the preparation of encapsulated toners are known. For example, a toner encapsulation process is illustrated in U.S. Pat. Nos. 4,727,011 and 4,877,706, the disclosures of which are totally incorporated herein by reference, which processes comprise, for example, 1) mixing a blend of a core monomer or monomers, free radical chemical initiator, pigment, and an oil soluble shell monomer; 2) dispersing the resulting mixture in a stabilized aqueous suspension; 3) thereafter subjecting the stabilized droplets to a shell forming interfacial polycondensation reaction by adding a water soluble shell monomer or monomers; 4) subsequently forming the core resin binder by heat induced free radical polymerization within the newly formed capsules; 5) washing the toner suspension to remove the surfactant in a filtration or centrifuging step; 6) diluting the resulting toner concentrates with water and adding a conductive colloidal graphite to the toner suspension prior to spray drying; 7) recovering the toner in a spray drying process; and 8) blending the recovered toner with conductive additive, release additive and flow aid. Another toner encapsulation process is described in U.S. Pat. No. 4,725,522, the disclosure of which is totally incorporated herein by reference, which substitutes the aforementioned first four steps with 1) dispersing pigment and magnetite in an organic solution of an elastomer; 2) adding shell monomers; 3) dispersing the resulting mixture in water containing a surfactant stabilizer; and 4) subsequently heating the reaction mixture to enable hydrolysis and an interfacial polymerization reaction thereby allowing the formation of a hard shell. These toner encapsulation processes employ a spray drying method to recover the toner from an aqueous suspension and to produce a free flowing toner powder. In U.S. Pat. No. 4,877,706 a conductive encapsulated toner composition is prepared by spray drying the toner suspension after adding a conductive component such as AQUADAG E.RTM. (Acheson Colloids Ltd.), a specially prepared water based dispersion of conductive colloidal graphite and a polymeric binder. The resulting toner may contain a layer of conductive graphite or carbon black uniformly and completely covering its surface. After drying, the toner is blended with a conductive additive, release additive or flow aid to produce a toner with a volume resistivity of about 1.times.10.sup.3 to about 1.times.10.sup.8 ohm-cm, and preferably from about 5.times.10.sup.4 to about 1.times.10.sup.7 ohm-cm, measurable in a 1 cm.sup.3 cell test fixture at 10 volts.
Spray drying is commonly employed to separate solid toner particles from an aqueous medium in many encapsulated toner processes. In one application, a toner suspension can be fed directly to a spray dryer to result in a free flowing powder. When toner washing is required to remove surfactant, the toner concentrate after filtration can be diluted and then fed to a spray dryer. As described in the prior art, a conductive coating can conveniently be applied to toner particles via the spray drying process. Economically, however, the spray drying process represents an expensive manufacturing method in comparison to other drying processes such as fluidized bed drying and vacuum drying. Typically, in spray drying processes a suspension containing 30 percent by weight of solid precursor toner particles is fed into a spray dryer. Thus, for every 3 parts of toner recovered 7 parts of water will have to be evaporated. With the fluidized bed drying or vacuum drying of the present invention in embodiments, a toner concentrate containing about 85 percent by weight of solid encapsulated toner particles are dried providing a toner to water weight ratio of approximately 5.7 parts to 1 part. The extra thermal energy required to evaporate water can render spray drying a more costly approach from, for example, a manufacturing point of view. In addition, for the same production capacity the physical dimensions of a spray dryer are much larger than either a fluidized bed dryer or a vacuum dryer, thereby adding to the capital inventment costs. Therefore, there is a need for replacing spray drying processes with more economical drying processes, such as fluidized bed drying, and a need for processes enabling the effective blending of additives, such as charge control agents, and the like to toners prepared by the drying methods indicated herein.
In a patentability search report there were recited the following U.S. Pat. Nos. 4,699,866 which discloses a process for the preparation of encapsulated toners followed by spray drying and heating; the heating can be accomplished in a fluid bed apparatus; U.S. Pat. No. 4,784,930 which discloses a process for the preparation of encapsulated toners wherein spray drying and heat drying in, for example, a fluid bed dryer and an infrared dryer are selected; U.S. Pat. No. 4,636,451 which discloses a process for the preparation of encapsulated toners wherein water removal is effected by spray drying, air drying, vacuum evaporation, centrifugal separation, and the like; and U.S. Pat. No. 4,599,294 which discloses a means for the drying of particulate materials, such as carbon black.
The in situ toner obtained with the processes of the present invention can be selected for a variety of known reprographic imaging processes including electrophotographic and ionographic processes. In one embodiment, the encapsulated toner can be selected for pressure fixing processes wherein the image is fixed with pressure. Pressure fixing is common in ionographic processes in which latent images are generated on a dielectric receiver, such as silicon carbide, reference U.S. Pat. No. 4,885,220, the disclosure of which is totally incorporated herein by reference. The latent images can then be toned with a conductive encapsulated toner by inductive single component development, and transferred and fixed simultaneously (transfix) in one single step onto paper with pressure. In another embodiment, the toner compositions can be utilized in xerographic imaging apparatus wherein image toning and transfer are accomplished electrostatically, and transferred images are fixed in a separate step by means of a pressure roll with or without the assistance of thermal or photochemical energy fusing. Also, an encapsulated toner obtained with the processes of the present invention can be selected, it is believed, for magnetic image character recognition (MICR) processes, reference U.S. Pat. No. 4,517,268 and U.S. Pat. No. 33,172, the disclosures of which are totally incorporated herein by reference.
In situ toners usually require surface additives such as charge control agents, release components, and flow aid materials. For example, one application of an encapsulated toner is in the known inductive single component development process. The toner material used in these processes usually possess high electrical conductivity at the outer surface of the toner particles. For example, for commercial ionographic printers such as Delphax S 9000.TM., S 6000.TM., S 4500.TM., S 3000.TM., Xerox 4060.TM. and Xerox 4075.TM. toners with a resistivity (the inverse of conductivity) of from about 1.times.10.sup.3 to 1.times.about 10.sup.8, and preferably from about 5.times.10.sup.4 to about 1.times.10.sup.7 ohm-cm, are selected. For encapsulated toner with a polyurea shell as described in, for example, U.S. Pat. No. 4,877,706, the disclosure of which is totally incorporated herein by reference, the toner without any additive coating has an electrically insulating surface. According to the teaching of this patent, the addition of certain colloidal graphite coatings on the toner surface during spray drying reduces the toner resistivity from about 10.sup.13 ohm-cm to about 10.sup.4 ohm-cm. Conductive carbon black and nonconductive release agent can be subsequently added to the encapsulated toner in a dry blending process to provide the toner with a resistivity of, for example, 5.times.10.sup.4 to 1.times.10.sup.7 ohm-cm. Thus, dry blending can be considered an important step in controlling the toner surface properties, which properties are of importance to print quality in, for example, xerographic copiers and printers.
Although the spray drying process is capable of producing a free flowing powder from a toner suspension, there remains a need for a simple, economical drying process for recovering in situ encapsulated toner compositions, wherein the toner particles are prepared in an aqueous phase. There is a need for an economical encapsulated toner manufacturing process wherein, for example, lower capital investment and operating cost are associated therewith as compared to present drying processes; for example, the capital cost saving can be up to about $1,000,000 annually when producing from about 1 to about 10 million pounds of toner with the operating cost saving being about 50 cents per pound of toner in some embodiments. There is also a need for processes that enable the coating of encapsulated toners with additives thereon that control the functional properties of the toner. There is also a need for processes wherein economical surface additives such as carbon black are applied in a simple dry blending operation, and wherein the need for surface coating during drying can be avoided.